A mobile communication apparatus terminal such as a mobile phone of a latest type increases not only an occupancy ratio of a liquid crystal display part to a terminal casing, but also the number of components mounted to the casing, as functions available as a mobile communication means further increase. Thus, a weight reduction, that is, a thickness reduction is required for the casing having greater weight of a mass that occupies in the whole terminal.
A plastic injection-molded product excellent in mass productivity is used for the casings of mobile phones that are on the market in large quantities among mobile communication apparatuses, in which case, however, various problems exist.
One of the problems is that the plastic material is inferior in mechanical properties such as tensile strength, modulus of elasticity and impact resistance as compared with a metal material, and besides, a residual stress at the time of molding causes a plastic molded product of a small thickness to be deformed so that degradations in thermal reliability occur, resulting in a limitation in the thickness reduction of the casing comprised of the plastic molded product. While use of fiber reinforced plastic (FRP) permits the mechanical properties and/or thermal reliability to be improved to some extent, it is difficult to manufacture a practically serviceable casing having a thickness of 1.5 mm or less using the fiber reinforced plastic.
Another problem is that the casing comprised of the plastic molded product fails to shield electromagnetic waves having undesirable effects on human bodies after leakage from a circuit of internally packaged electronic components, resulting in a need for electromagnetic wave shielding works by giving surface treatments etc. in the manner of plating such as ion plating and electroless plating with copper and nickel etc.
In view of the above backgrounds, a casing obtained by giving forging to an aluminum alloy blank (a rolled sheet) has been suggested in recent years (See Patent document 1 described later). Specifically, an aluminum alloy has a higher density of 2.7 g/cm3 as compared with 0.8 to 1.4 g/cm3 for the plastic material and is also about 1.7 times the tensile strength and about 6 times the modulus of elasticity in comparison with the plastic material. Thus, the aluminum alloy ensures that a specific strength (tensile strength/density) and a specific rigidity (modulus of elasticity/density) are relatively high, permitting contribution toward the thickness reduction of the casing.
The casing involving use of the aluminum alloy is effective in absorbing or reflecting the electromagnetic waves, and thus provides a higher electromagnetic wave shielding efficiency as compared with a casing obtained by giving plating to the plastic material. In the present days with a tendency to call for more strict regulations for the control of electromagnetic troubles, an advantage that the casing itself has the electromagnetic shielding efficiency is by no means negligible.
Further, the aluminum alloy casing has other advantages that anodic oxide coating may provide an impression of higher grade for the casing, and besides, is excellent in recycling efficiency.
However, the casing obtained by giving forging to the aluminum alloy rolled sheet gives rise to problems such as degradations in dimensional precision and fluctuations in surface form and properties. That is, in the case of hot forging, an anisotropy of a material strength depending on a metal forging flow direction and the dimensional precision arises. In the case of cold forging, though somewhat improved dimensional precision is provided, there are problems that formability and dimensional precision are inferior as compared with punching and cutting works, and a surface form and properties after forging are anything but desirable, in addition to a reduction in degree of freedom in selection of forms.    Patent document 1: Japanese Patent Laid-open No. 2002-64283